I have empirical data that suggests an increase of temperature of 4 degrees C when a diffuser is fitted to the rear of a Se7en. Diff temps are therefore *not* an issue when considereing a rear diffuser.
My car has had one for it's whole life. I like it. Looks trick, keeps the rear suspension and diff clean, and add's about half a kilo of downforce 'cos it's made of pre preg carbon/honeycomb material, and not lead